1. Field of the Invention
This invention relates to a latex and the method for the preparation thereof. More particularly, it is concerned with a concentrated latex of synthetic rubbers and the method for the preparation thereof.
2. Description of the Piror Art
As exemplified by styrene-butadiene copolymer latex, synthetic rubber latex is usually prepared by emulsion polymerization and widely used in practice. On the other hand, progress of polymerization technique has recently produced a variety of characteristic solution-polymerized synthetic rubbers, which have been made attempts at the use in latex. General procedures for the preparation of latex from such a solution-polymerized synthetic rubber involve dissolving a solution of polymerized rubber or a solid rubber in an appropriate solvent, adding an emulsifier and water to the solution followed by emulsification and removing the solvent from the emulsion by stripping, flushing or distillation. The latex thus obtained, which is a so-called dilute latex, should then be concentrated for conventional use by such a process as creaming or centrifugal separation.
One of the typical synthetic rubbers obtained by solution polymerization is block copolymers in which an elastic polymer block and a non-elastic polymer block are alternately arranged. They are represented by A-B-A, (A-B)n, B-(A-B)n, A-(B-A)n and A-B-(B-A)n wherein A is a non-elastic polymer block usually having a second order transition temperature of 25.degree. C. or higher, B is an elastic polymer block usually having a second order transition temperature of 10.degree. C. or lower and n is an integer 2 or more, which are called thermoplastic elastomer and usually have large strength and elongation and a low residual strain comparable to the ordinary vulcanized rubber, without having been subjected to bridging, for example, with a sulfur compound.
Progress of polymerization technique in recent years has also enabled production of a variety of monovinyl aromatic hydrocarbon-aliphatic diene copolymers by means of a solution polymerization process. For example, whereas styrene-butadiene copolymers made by emulsion polymerization contain only randomly combined styrene and butadiene, there are produced in the anion living polymerization, for example, employing an organolithium compound, copolymers ranging from one with styrene and butadiene randomly combined to one respectively with styrene and butadiene blockly combined. Whereas bonding of the butadiene moiety in emulsion-polymerized styrene-butadiene copolymers is mainly composed of trans 1,4-bond, it is possible in the solution-polymerized copolymers to modify bonding of the butadiene moiety in a wide range depending upon the solvent and the catalyst selected, thereby producing one with predominant trans 1,4-bond or one mainly with 1,2-vinyl bond. Thus, unlike emulsion polymerization for styrene-butadiene copolymers, it is possible according to the solution polymerization to produce styrene-butadiene copolymers of widely varied structures with a variety of physical characteristics. For example, vulcanization product from rubber using a styrene-butadiene random copolymer produced by the polymerization in a hydrocarbon solvent in the presence of an alkyllithium catalyst is excellent in impact resilience and abrasion resistance; rubber compound using a styrene-butadiene block copolymer made by a solution polymerization process is characterized by a high hardness and excellent low-temperature properties.
Latex of the block copolymers represented by the abovementioned general formulae and latex of the aforesaid solution-polymerized styrene-butadiene copolymers are utilized in a wide variety of uses such as dip molding, cast molding, form rubber, rubber thread, finished paper, carpet-sizing agent, fiber-processing agent, surface-coating agent, adhesive, paint binder, latex-containing asphalt and cement-compounding ingredient.
However, it is not easy to produce latex of this nature in concentrated state. In preparing concentrated latex from solid rubber by the method as set forth above, prevention of precipitation or coagulation of rubber by aggregation of particles during the production is generally a matter of fundamental importance. Such precipitation and coagulation could occur under various conditions, for example, in association with change in external conditions such as heat or mechanical pressure. They are liable to be noticeable especially when the dilute latex is concentrated by a process such as creaming or centrifugal separation. However, as disclosed in Japanese Patent Publications 20430/1964, 12626/1966 and 34261/1970, the problem will not be critical or can be solved by selection of an appropriate emulsifier in the production of concentrated latex from most of solid rubbers such as polyisoprene, isobutylene-isoprene copolymers, ethylene-propylene copolymers and ethylene-propylene terpolymers.
On the other hand, although the above-mentioned block copolymers and the above-mentioned styrene-butadiene copolymers can be processed in the same way as other solid rubber to a dilute latex, concentration of said latex tends to be associated with much precipitation or coagulation so that not only the production will be much reduced but also there will be produced only a product with a very low mechanical stability.
More particularly, a good emulsion is produced when a solution of the above-mentioned copolymers is emulsified by means of an emulsifier in the presence of an emulsifying agent selected from the following members; Anionic surface active agent including fatty acid salt soaps or rosin acid soap such as potassium oleate, sodium laurate, potassium palmitate and sodium rosinate, or higher alcohol sulfate salts such as sodium octylsulfate and sodium lauryl-sulfate, alkylbenzenesulfonate salts such as sodium dodecylbenzenesulfonate and sodium octylbenzenesulfonate and aliphatic alcohol phosphate salts such as sodium octyl alcohol phosphate, or cationic surface active agents including aliphatic amine hydrochlorides such as octylamine hydrochloride, dodecylamine hydrochloride and lauryl-methylamine hydrochloride, quaternary ammonium salts such as octyltrimethylammonium bromide, dioctyldimethylammonium chloride and benzyldimethyloctylammonium chloride and alkylpyridinium salts such as dodecylpyridinium chloride and hexadecylpyridinium chloride. During the subsequent step for removing the solvent from said emulsion by distillation, however, 5-10% (in the case of the above-mentioned block copolymers) or 3-8% (in the case of the above-mentioned styrene-butadiene copolymers) in terms of the initially charged copolymer of the particles will be precipitated or coagulated. Moreover, during the step for concentrating the dilute latex obtained by the solvent removal by means of creaming or centrifugal separation, 80-100% (in the case of the above-mentioned block copolymers) or 20-60% (in the case of the above-mentioned styrene-butadiene copolymers) of the particles will be precipitated or coagulated so that it is absolutely difficult to obtain a concentrated latex.
The low mechanical stability for latex of the above-mentioned block copolymers is not clearly understood for mechanism. However, according to Japanese Patent Publication 27738/1965, it is described that the low stability is due to the shape of fine particles produced on emulsification which form non-spheroid, for example, discoid or acorn owing to strong internal orientation characteristic of the block copolymer of this nature. There is disclosed in said patent publication a method for prevention of the precipitation or coagulation comprising treatment of the dilute latex with a liquid aliphatic hydrocarbon prior to concentration. The method involves steps of adding to a dilute latex of the block copolymer a liquid aliphatic hydrocarbon being solvent only for the elastic blocks but not for the non-elastic blocks at a ratio by weight from 1:1 to 5:1 is terms of said hydrocarbon to said block copolymer, effecting the contact for at least 1/4 hour, then removing the aliphatic hydrocarbon and subsequently effecting the concentration. It is stated that such a treatment produces particles in nearly spheroidal form and increases the stability to a degree sufficient to be concentrated. However, the method is not satisfactory in that it is time-consuming and complicated in procedure and more effective methods are desired to be developed.